Presently, a major portion of point-to-point data communication between data terminal equipment ("DTEs" or terminals), such as personal computers and workstations, is implemented using data communications devices ("DCDs"), such as analog modems, digital modems, and ISDN terminal adapters. Most such data communications is currently over analog communications systems, such as the public switched telephone network ("PSTN"). Currently, the fastest high-speed analog modems operate at data transfer rates of approximately 28.8 kbps (kilobits per second) to over 30 kbps.
A variety of desirable applications, such as videoconferencing, downloading data files from various networks, and transmitting documents or other data to various networks or other DTEs, may require higher speeds of data transmission than is currently practicable, affordable, or available over most analog PSTN systems. Such higher speeds for data transmission may also be preferrable for consumers, as slower data speeds may result in delays which are perceived as commercially unacceptable. Accordingly, many telecommunication providers have begun to offer and to implement various digital and other high speed data transmission services to meet the corresponding demand for greater data transmission capability, higher data transfer rates, and greater data transmission bandwidth, such as switched digital services, T1 services, E1 services, Integrated Services Digital Networks ("ISDN"), and the emerging Motorola CableComm.TM. system utilizing hybrid fiber coaxial cable. A typical T1 connection has a data rate of 1.544 Mbps (megabits per second), which is further time division multiplexed ("TDM") into digital (not analog) transmission channels. The T1 connection comprises 24 channels, with each channel referred to as a DS0 having a data rate of 64,000 bps. The emerging Motorola CableComm.TM. system may have data transmission rates as high as 30 Mbps. Specifications and standards for ISDN, T1, and E1 services are described in numerous International Telecommunications Union ("ITU") Recommendations, such as Recommendation G (for T1, E1), and the I-Series Recommendation (for ISDN). Other discussions of analog and digital telecommunications services may be found in a wide variety of references, such as R. Freeman, Reference Manual for Telecommunication Engineers, John Wiley & Sons, 1985.
For ISDN, a typical ISDN Basic Rate Interface ("BRI") service currently available to end users consists of three time division multiplexed channels, comprising two DS0s known as B channels operating at 64 kbps and one D channel operating at 16 kbps. The B channels may be used for transmission of voice, video, multimedia, and other data and information (individually and collectively referred to herein as "data"), while a D channel may be used for packetized data and/or control information (i.e., call set-up and shut down), with additional bandwidth utilized for framing, synchronization, and other overhead bits. A DCD such as an ISDN terminal adapter ("TA") may be used for transferring data between DTEs (such as personal computers, terminals, local area networks, video conferencing equipment and similar equipment) using such ISDN service. While two B (data) channels (each operating at 64 kbps) are sufficient for many applications, there are applications in which one data channel having a higher data bandwidth is preferable. As a consequence, a process known as "bonding" (Bandwidth ON Demand) has evolved, in which two or more channels (each having lesser bandwidth) are concatenated or combined into one channel having having greater bandwidth. For example, it may be desirable to combine the two, 64 kbps B channels to obtain a single bonded channel operating at 128 kbps. The 128 kbps channel formed by bonding may then be used to transfer data between two DTEs. Depending upon the service provider, additional DS0s may also be combined to create channels operating at even higher data rates. In addition, higher data rates are or will be available with other communication systems, such as the emerging Motorola CableComm.TM. system mentioned above.
Various difficulties with operation at these higher data rates, however, may arise from the typical, standard hardware and software resident in DTEs such as personal computers ("PCs"), servers, routers, bridges, and other computer and communication networking equipment. For example, for a personal computer, data transfer typically occurs across a 16 bit peripheral interface or bus, such as the Industry Standard Architecture bus commonly known and referred to as an "ISA" bus, or across another standardized bus or interface format known as a PCMCIA (Personal Computer Memory Card International Association) bus or interface. Data is transferred between the PC and a connected TA (stand alone or internal PC card), for ISDN or other digital transmission, via an ISA or PCMCIA bus or interface. Typical terminal adapters and other data communications devices have employed Universal Asynchronous Receiver Transmitters ("UARTs") for serial data transmission and reception over an ISA bus or other interface. In addition, many data communications applications programs, such as those utilizing a Windows.RTM. operating system, have been developed presuming that the underlying hardware will include a UART for data transfer and, accordingly, utilize drivers (software drivers) designed for UARTs. Most UARTs, however, are only capable of a maximum data transmission speed of 115.2 kbps, which is acceptable for most applications, but cannot be used to take full advantage, for example, of bonded ISDN channels operating at 128 kbps or or other communication media or network connections operating at even higher data rates. Use of other UARTs, which may operate at higher data rates, may be prohibitively expensive for inclusion in terminal adapters or other data communications devices, and, again, may also be insufficient for very high speed transmission (such as that available over several bonded channels). As a consequence, in digital communications systems, use of a typical UART within a data communications device may create an unwanted "bottleneck" situation, slowing down data transmission in circumstances where higher data transmission rates are otherwise possible and desirable.
Accordingly, a need has remained for an apparatus and method to provide for higher speed data transmission over communication interfaces or busses, such as ISA busses, in order to utilize the full transmission capability which may be available over digital or other communication systems. In addition, a need has remained to provide for such higher speed data transmission cost effectively and with ease of manufacture. Lastly, a need had remained to provide for such higher speed data transmission either utilizing or retrofittable with existing computer architecture and operating systems, such as the Macintosh.RTM. and the various Windows.RTM. operating systems for personal computers.